Method and equipment for taking a sample

ABSTRACT

The invention concerns a method and an equipment for taking a sample in slurry polymerization. The sample is taken directly from the liquid phase of the reactor (15) through an in-line filter (40). The slurry flow is maintained in the in-line filter (40) at a sufficiently hight level to prevent immobilization of polymer or catalyst particles on the filter face of the in-line filte (40).

The invention concerns a method and an equipment for taking a sample inslurry polymerization.

Various methods have been developed for the preparation of solid andsemisolid polymers out of hydrocarbons, such as 1-olefins. In one suchmethod, olefins, such as ethylene, propylene, butylene, or pentenes, arepolymerized in the presence of catalysts in hydrocarbon diluents orwhile the monomers themselves act as diluents. In such a case, thereaction agents are kept in the solution phase by maintaining a suitablepressure in the polymerization reactor. When the polymer that is formedis insoluble or poorly soluble in said diluent, the polymer product isformed as particles and, thus, the product flow comprises a suspensioncomposed of polymer particles, diluents, and monomers. This product flowis usually passed into a polymer separation tank, in which the solidmaterials and the liquid and gaseous constituents are separated from oneanother.

One reactor type that is applied in such methods is a continuous pipereactor that forms a loop, the polymerization taking place in aturbulent flow circulating in the loop. The product, which contains apolymer, diluents and monomers, is taken out of the loop reactor eithercontinuously or, more commonly, periodically through an exhaust valve,being passed into a separator, in which the polymer is separated bylowering the pressure.

In view of regulation of the polymerization reaction, samples can betaken from the product flow of the reactor continuously or periodically.A normal way is to take a sample from the gas flow departing from thepolymer separation tank and to analyze this gas sample by variousmethods, for example by means of gas chromatography. Such an arrangementis described, e.g., in the U.S. Pat. No. 3,556,730.

In this prior-art procedure, the period of delay that occurs from thedeparture of the product from the polymerization reactor until the timeof start of the analysis is often considerably long, and during thatperiod essential changes may take place in the process. Thus, the sampleis not representative at all times. Therefore, it would be preferable tomake said period of delay of analysis shorter.

In the FI Patent No. 85,191, a method is described by whose means asubstantial shortening of the sampling delay is achieved. In saidmethod, the sample is taken from the product pipe through a closingvalve of the on/off type, which valve is closed for the time of thepressure swing produced on opening of the exhaust valve, and which valveis opened after said pressure swing.

In a polymerization plant, the process delay related to the analysis is,as a rule, of an order of about 10 minutes. When a sampling system inaccordance with the FI Patent 85,191 is used, the process delay relatedto the analysis is shortened to about one minute under equivalentconditions.

For the regulation of the properties of the polymer in slurrypolymerization, precise regulation of the concentration is required.Conventionally this has been carried out by analyzing the inputconcentration, in mixing-tank reactors the gas-phase concentrations orthe concentrations of the gas phase separated from the product. Thesemeasurements do, however, not give sufficiently precise information onthe liquid-phase concentrations in the reactor.

When the input concentration is analyzed, the factual concentrations inthe reactor are not known. When the gas-phase concentrations in amixing-tank reactor are analyzed, the factual liquid-phaseconcentrations are not known. Further, problems are produced by theadhesion of polymer particles in the sampling system. When the taking ofsamples takes place from the gas flow after the product pipe of thereactor (after the separation tank), adhering of polymer particles inthe sampling line causes problems. Further, possible return-blow gasesin the product filters may interfere with the analyzing, and there maybe a long time delay until the analysis of concentration has beencarried out. Also, analyzing from the gas space is impossible if thepolymer slurry is fed from one reactor into another without separationof gases. An analysis directly from the slurry by means of aconventional filter is not possible, because the filter is underreaction conditions and, if small catalyst particles remain on the faceof the filter, said particles continue the polymerization on the filterand block the filter rapidly.

The object of the present invention is to provide a method and anequipment which permit taking of samples without any substantial delaysand in which the problems arising from adhering of polymer particleshave been eliminated. It is a further object of the invention to providea method and an equipment which may also be applied to slurry reactorswhich are connected in series and in which there is no separation ofpolymer and liquid/gas between the reactors.

The method in accordance with the invention is characterized in that thesample is taken directly from the liquid phase of the reactor through anin-line filter, and that the slurry flow is maintained in the in-linefilter at a sufficiently high level to prevent immobilization of polymeror catalyst particles on the face of the in-line filter.

The equipment in accordance with the invention is characterized in thatthe equipment comprises an in-line filter, which communicates with theliquid phase of the reactor by one of its ends by means of a first flowduct and by the other one of its ends by means of a second flow duct.

The method of analysis in accordance with the invention is carried outdirectly from the liquid phase in which the polymerization takes place,without any time delay and without problems of adhesion of particles.The in-line filter in accordance with the invention can also beaccomplished with a minimal number of moving and wearing parts.

In the method in accordance with the invention, the in-line filter iswashed by means of a slurry flow of a very high velocity, e.g. 3 . . .10 metres per second. In the case of the slurry-loop reactor, thedifference in pressure across the pump of the loop reactor produces thenecessary high-velocity flow in the pipe of the filter. In the case of atank reactor, it is possible to use a slurry pump. If necessary, it isalso possible to use reflux washing with an inert liquid to cleanse thefilter.

The invention will be described in detail with reference to somepreferred embodiments of the invention illustrated in the figures in theaccompanying drawings, the invention being, however, not supposed to beconfined to said embodiments alone.

FIG. 1 is a schematic illustration of a conventional prior-anloop-reactor system.

FIG. 2 is a schematic illustration of a prior-an sampling system inaccordance with the FI Patent 85,191.

FIG. 3 is a schematic illustration of a preferred embodiment of thesampling method in accordance with the present invention.

FIG. 4 is a schematic illustration of a second preferred embodiment ofthe sampling method in accordance with the invention.

In FIG. 1 (prior art), the reference numeral 10 represents apolymerization equipment, in which, through a feed pipe 11, a monomer ispassed from the pipe 12, a catalyst from the pipe 13, and a diluent fromthe pipe 14 into the loop reactor 15. In the reactor 15 pipe 16, thesuspension consisting of the reaction agents and of the polymer that isbeing formed is circulated at a high velocity by means of a circulationequipment not shown, such as a pump or a propeller device. Thetemperature in the reactor 15 can be regulated by means of aheating/cooling mantle 17. The above system of feed of the reactionagents is just suggestive, and so the reaction agents can be passed intothe reactor 15 in any desired way whatsoever, together or separately.

Out of the reactor 15, the suspension consisting of polymer, diluent,and monomer is removed through the valve 18. The valve 18 is openedperiodically for a short time, for example, at intervals of half aminute, and allows product suspension to pass through the pipe 19 intothe separation tank 20. In the separation tank 20, owing to the loweringof the pressure, the diluent contained in the suspension is gasified,the solid polymer product being removed through the pipe 21, and thegaseous phase which contains diluent and monomer is removed through thepipe 22, and said phase can be returned into the reactor 15 along thepipe 24 after its pressure has been increased in a compressor 23.

Out of the pipe 22, through the pipe 25, a gas sample can be taken tothe analyzer 26. Since the volume of the separator 20 is considerablylarge and since the product flow into the separator is periodic,adequate mixing of the-gas phase and formation of a representativesample take a considerable length of time, which may be up to 15minutes. Thus, this causes a considerable delay in the analysis.

FIG. 2 (prior art) shows a sampling system in accordance with the FIPatent 85,191, wherein the sample is taken from the reactor 15 throughthe duct 27 and the exhaust valve 18 relatively soon directly from theproduct pipe 19 through the valve 28 into the pipe 31 passing to theanalyzer, through a constant-flow valve 30.

When the exhaust valve 18 is opened, the pressure of the reactor 15 hasaccess directly into the product pipe 19, and this produces aconsiderable pressure shock. This pressure shock must not have access tothe analyzers, and therefore the valve 28 that is used is a valve of theon/off type, which valve is closed when the exhaust valve 18 is opened,for the time of the maximum pressure swing. The duration of the pressureswing is of an order of 5 seconds. After the pressure swing, the valve28 is opened and allows the sample to flow to the analyzer through thevalve 30 and the pipe 31.

In order that the sample flow to the analyzer should be continuous,after the valve 28 a buffer tank 29 is provided, whose volume has beenmeasured such that it is sufficient to guarantee that the flow throughthe pipe 31 is continuous even if the valve 28 is closed in between.

In the embodiment as shown in FIG. 3, the taking of a sample takes placein accordance with the basic realization of the invention directly inthe liquid phase of the reactor 15 by means of an in-line filter 40. Oneend of the filter 40 communicates with the pipe 16 of the loop reactor15 through the flow duct 41, and the other end of the filter 40 throughthe flow duct 42. The flow duct 41 includes a valve 43, and the flowduct 42 includes a valve 44. The sample is passed from the filter 40through the duct 45 to gas-chromatographic analysis. The referencenumeral 46 represents a vaporizer and pressure reducer. The sample flowsfrom the pipe 16 of the loop reactor 15 through the pipe 41, through thein-line filter 40, and through the pipe 42 in the direction indicated bythe arrow A at a very high velocity. A suitable velocity is, as a rule,3 . . . 10 metres per second. The circulation equipment placed in thepipe 16 of the loop reactor 15 is denoted with the reference numeral 32.

In the embodiment as shown in FIG. 4, one end of the in-line filter 140communicates with the slurry tank reactor 115 through the flow duct 141,and the other end of the filter through the duct 142. The flow duct 141includes a slurry pump 143. The sample is passed from the filter 140along the flow duct 145 to gas-chromatographic analysis. The agitatordevice placed in the slurry tank reactor 115 is denoted with thereference numeral 132.

We claim:
 1. Method for taking a sample from the liquid phase of a loopreactor (15) in slurry polymerization through an in-line filter (40)placed in the liquid phase flow of the reactor, characterized in thatthe slurry flow across the in-line filter (40) is maintained at asufficiently high level to prevent immobilization of polymer or catalystparticles on the filter face of the in-line filter (40) utilizing thepressure difference produced by a circulating pump (32) of the loopreactor (15).
 2. Equipment for taking a sample from a loop reactor (15)in slurry polymerization, characterized in that the equipment comprisesan in-line filter (40) which communicates with the slurry, which flowsin a pipe (16) of the loop reactor (15), through a first flow duct (41)at one end thereof and through a second flow duct (42) at the other endthereof, and that the first flow duct (41) communicates with thepressure side of a circulating pump (32) of the loop reactor (15) andthe other flow duct (42) communicates with the suction side of thecirculating pump (32) of the loop reactor (15).
 3. Equipment as claimedin claim 2, characterized in that the first flow duct (41) is providedwith a valve (43) and the second flow duct (42) is provided with a valve(44), and that the in-line filter (40) communicates through a duct (45)with gas-chromatographic analysis.